The present invention relates to the supply of steam under conditions of pressure and temperature to a steam turbine.
The admission of steam to a steam turbine raises a problem of matching the temperature of steam with the temperature of the turbine in order to avoid stresses therein, especially in the rotor thereof. At the same time, efficiency of utilization of the steam and of the turbine require that such matching be achieved promptly, thereby to minimize the lag between a cold steam input at the restart and a hot turbine rotor, or between a hot steam input and a cold turbine rotor, and both to minimize rotor stress and plant startup time.
Control system are known which permit to ascertain rotor stresses under start-up and/or under load. Such systems modify the operation of the turbine whenever a critical limit in the rotor stresses is encountered. It is also known to operate a turbine at a maximum level of stress and strain short of the unacceptable limit, thereby to maximize the utilization of steam in the system.
The prior art makes use of temperature measurements effected at various locations, such as where the steam flows, where the rotor is exposed to temperature gradients under the steam, where the true temperature of the rotor manifests itself. Measurements as well as calculations of heat flow have been used to ascertain the extent of heat transferred to the rotor as a function of time. More generally, the prior art has been concerned with establishing the temperature of the rotor at any given time and the state of the steam influencing rotor temperature at any given moment whereby rotor stresses can be and have been calculated, thereby to instantaneously ascertain how close start-up or loading of the turbine is to bring about such rotor stresses and strain as would exceed a predetermined critical level.
More specifically, U.S. Pat. No. 3,558,265 (Berry) discusses the effects of thermal loading on a turbine, the risks of rotor thermal stress and plastic strain which affect the life of the turbine due to rapid gradients of temperature when hot steam is first admitted upon the rotor, the casing and blades, as well as under the turbine dynamics in normal operation after start-up. Present rotor stress is determined as a function of steam temperature within the turbine and steam flow at the inlet of the turbine is controlled in relation to how far the system is from limit stress.
U.S. Pat. No. 3,446,224 (Zwicky) proposes to calculate present surface and bore stress of the rotor from measured steam temperature and rotor speed, thereby to prevent exceeding critical limits of stress.
U.S. Pat. No. 3,928,972 (Osborne) proposes to control the dynamic operation of a steam turbine as a function of heat flow, whereby a process variable is used which is determinative of future stress. Calculated heat flow is compared with a reference heat flow which provides maximum rotor strain without exceeding the limit, whereby the desired changes in the turbine operation are accomplished by the shortest possible sequence, while minimizing the lag between steam temperature changes and turbine temperature changes. This approach insures rapid starting of the turbine and allows rapid load changes without damage to the rotor core and blades of the turbine. The Osborne patent makes use of turbine casing temperature, impulse chamber steam temperature and impulse chamber steam pressure which are sensed and combined for load control. The throttle pressure is detected for the purpose of maintaining constant throttle pressure in the supply of steam to the turbine.
It is known from the aforementioned Zwicky patent to sense with thermocouples temperature of the inner casing as an indication of steam temperature. This is used during start-up, together with an indication of speed to calculate simulated surface and bore stress values to be compared with stress limits. The stress margin so derived is used as a corrective signal when positioning the steam turbine valve.
In the Osborne patent, rotor temperature is indicative of the heat buildup in the turbine. Progression of the heat build-up is controlled by controlling the heat flow, and such progression is determined by the distance, or closeness, to a predetermined limit to be approached as quickly as possible without exceeding it.
The prior art also proposes to control the steam supply to the turbine in order to obtain a controlled gradient of temperature on the rotor of the turbine.
As part of the start-up process for thermal stress protection, as explained in U.S. Pat. No. 4,029,951 (Berry), a heat soak period is established for a period of time while running the turbine at reduced speed. Once the soak time is completed, the turbine is allowed to be accelerated to synchronous speed.
In U.S. Pat. No. 3,928,972 (Osborne), consideration is made of a normal transient condition for start-up or acceleration, according to which the temperature gradient through the rotor is substantially constant and under which the heat transfer coefficient changes with speed in such a way that change of speed causes a heat flow change. A similar transient condition is experienced at steady state, when the turbine load is increased, or decreased, at synchronous speed.
Finally, U.S. Pat. No. 4,005,581 (Aanstad) refers to the prior art approach of modulating the flow of steam to the turbine and distinguishes constant throttle pressure control from sliding pressure control during different portions of the turbine operating load range. In contrast, in the Aanstad patent, an operating representation is generated as a function of the turbine steam state to control steam flow. More specifically, such representation is the steam enthalpy derived on the basis of steam pressure and temperature, respectively, at the inlet and exhaust of the turbine stages. While so doing, the enthalpy drop due to steam expansion through the turbine is the parameter used for modulating steam flow.